In the semiconductor processing, liquid HF mixtures are often used for etching thick SiO.sub.2 layers or for etching chemical or native oxide layers.
For etching thick SiO.sub.2 layers, usually buffered HF is used, which is a mixture of NH.sub.4 F and HF, in the liquid phase, generally as a solution in water.
Moreover, as a last step in the cleaning process, usually a diluted HF step is used.
Etching of bare silicon wafers in HF solution and/or a final etch in HF solution generally in combination with the RCA-process have been the object of many experimentation and publications.
The RCA-cleaning process for bare or oxidized silicon wafers is based on a two-step oxidizing and complexing treatment with hydrogen peroxide solutions: an alkaline mixture at high pH and followed by an acidic mixture at low pH (W. Kern and D. Puotinen, "Cleaning Solutions Based on Hydrogen Peroxide for Use in Silicon Semiconductor Technology", RCA Review, 31, 187-206 (1970)).
"HF-last cleaning" result in a Si-surface free of silicon oxide and passivated with hydrogen. However, the execution of this process with liquid HF mixtures is very susceptible to contamination.
The etch bath, chemicals, rinsing water and air ambient need to be very clean to achieve good results. Otherwise, severe contamination of the wafers with particles, metals and organic material can occur during the etching rinsing or drying process. "HF-last" processing with liquid mixtures before gate oxidation is, therefore, still a point of controversy.
As an alternative, the etching can be performed with HF mixtures in the vapor phase to prevent recontamination from the liquid and during rinsing and drying of the wafers. By etching in the gas phase, recontamination from the liquid is omitted and the high susceptibility of the wafer for contamination during the drying stage is avoided. Furthermore, in the gas phase the etching of small features is facilitated while surface tension effects hamper this in the liquid phase.
Traditionally, the HF vapor etching is performed with a mixture of HF and H.sub.2 O vapors. This is described in U.S. Pat. No. 4,749,440 of FSI for processes performed at near atmospheric pressures in a mode where the process gases are continuous flowing, the so-called dynamic mode. Because of controllability problems with this process, the process was improved by performing it at substantially reduced pressures (600 Pa-2000 Pa) and applying a different procedure, the so-called static mode (see also U.S. Pat. No. 5,167,761). Despite of these improvements, the controllability of the process is still problematic.
As an alternative, a method for etching silicon oxide by feeding anhydrous HF and alcohol vapor simultaneously into a reaction chamber is described in U.S. Pat. No. 5,022,961. However, this process suffers from the same limitations as the HF/water vapor process.